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Amp Knockdown Switches

PostPosted: Thu Dec 29, 2005 8:05 am
by tomatocan
Hey everybody, I was wondering if anybody knows anything about wattage knockdown switches for amps. A guy I play with has one that brings his Twin amp down to 22 watts. I'd like to get one for my Bassman, because it's so damn loud. Any help, info, or recomendations would be great.


Pat
_____

http://www.tomatocanmusic.com

RE: Amp Knockdown Switches

PostPosted: Thu Dec 29, 2005 2:08 pm
by ricochet
There are various ways of modifying them to reduce the power, with things like cutting out 2 tubes of a 4 tube set, "triode switches" that make the beam tetrode or pentode output tubes work as triodes with the plates and screen grids hooked together (cuts the output stage power and gain way down), dropping the voltage applied to the output tubes... None of it's super-easy stuff, and all will change the amp's sound siginificantly. The best way is to buy or build a power soak to go between the amp and speakers, if you want it to sound "cranked" at a lower volume.

"A cheerful heart is good medicine."

RE: Amp Knockdown Switches

PostPosted: Thu Dec 29, 2005 5:02 pm
by bosco
The best way is to buy or build a power soak to go between the amp and speakers, if you want it to sound "cranked" at a lower volume.

That would interest me as well for my Bassman reissue, (50 watts), how would one go about that Rico? I get marvelous harp tone when I can crank it but it mostly sees outdoor only use. Also, I currently have the preamp diminished to 40% with a 12AY7, but that is the harper's standard easy way out and probably robs components of the optimum current they need to operate as designed (a guess). If a power soak was added, would you want to retube the preamp back to stock? (100%). Thx.

Bosco

RE: Amp Knockdown Switches

PostPosted: Thu Dec 29, 2005 5:42 pm
by ricochet
Simplest thing, of course, is to buy something like THD Hot Plates.

If I wanted to build one, I'd use light bulbs as the load. They're cheap, readily available and work. Size of bulbs will need some guesstimation. Figure the resistance of different wattage bulbs when hot from their voltage and wattage ratings. Their cold resistance is about 1/10 that. Because their resistance goes up as they heat up, when in series with the speaker they're volume compressors, and when they're bridged across the speaker in parallel they're volume expanders. Different combinations of the two can be used. Power and resistance calculations give you a place to start, then you've got to experiment to find out what you like. Don't forget to include the impedance of the speaker in your circuit for the calculations. And make sure a bulb blowing out can't suddenly unload your amp's output, as it would with a single bulb in series with the speaker. That'd fry your output transformer.

Somebody's likely done the calculations and posted some designs for typical ratings on the Web, but I haven't looked. Saw an amp for sale a while back that had a bulb on the chassis as the optional power reduction. Came from one of the boutique amp companies, can't remember which. It's an old idea, anyway. Some old Hammond organs use a bulb as a power limiter in the reverb driver amp.

"A cheerful heart is good medicine."

RE: Amp Knockdown Switches

PostPosted: Thu Dec 29, 2005 5:50 pm
by rustyslide
I looked at buying an attenuator for my 100W (that's 100W clean) amp, as I would love to send it into power-amp-distortion territory, but all the ones I could find on the 'net were quite expensive.

I can't pull tubes out as it's using FETs in the power amp.


I think I'm going to look into recreating a simple 12AX7/EL84 or 12AX7/6L6 (ie, one of each) amp circuit with FETs and sticking it in a very portable but nothing-special practice amp (that's also way too loud) I bought in the fall.

RE: Amp Knockdown Switches

PostPosted: Thu Dec 29, 2005 5:54 pm
by ricochet
>I think I'm going to look into recreating a simple
>12AX7/EL84 or 12AX7/6L6 (ie, one of each) amp circuit with
>FETs and sticking it in a very portable but nothing-special
>practice amp (that's also way too loud) I bought in the
>fall.

That's certainly doable, but you'll have to have the heavy and expensive parts of the tube amp (the transformers) if you're going to recreate such a tube amp circuit. Might as well do it with tubes and be done. Or buy a Valve Junior, which has got the single 12AX7 and EL84, and costs around $120 US.



"A cheerful heart is good medicine."

RE: Amp Knockdown Switches

PostPosted: Thu Dec 29, 2005 6:21 pm
by randy101
I just looked at the Marshall Power Brake.

$519.00!!

This would give someone a good reason to buy another amp.

RE: Amp Knockdown Switches

PostPosted: Thu Dec 29, 2005 7:14 pm
by bluesmcgoo
Leave the Marshall Power Break alone.

Get a Hot Plate...

http://www.thdelectronics.com/products/hotplate.htm

http://www.samedaymusic.com/product--THDHP

http://www.americanmusical.com/sort--s-hot-plate.html

They work about as good as it gets...keeping in mind it's an attenuator.

RE: Amp Knockdown Switches

PostPosted: Thu Dec 29, 2005 10:25 pm
by randy101
yeah but....I always need a new amp.

Or an excuse to buy something new

RE: Amp Knockdown Switches

PostPosted: Thu Dec 29, 2005 11:53 pm
by ricochet
Yeah. Buying a little amp's a better solution than putting an attenuator on a big one.

"A cheerful heart is good medicine."

RE: Amp Knockdown Switches

PostPosted: Fri Dec 30, 2005 1:38 am
by tomatocan
I do already own a 2ohm Hotplate for my Super Reverb, and it works great. But the bassman doesn't have the 1/4" speaker inputs and outputs that the Blackfaces have. Think I can get some installed?

Pat
_____

http://www.tomatocanmusic.com

RE: Amp Knockdown Switches

PostPosted: Fri Dec 30, 2005 1:59 am
by ricochet
Yep. It's not a big job.

"A cheerful heart is good medicine."

RE: Amp Knockdown Switches

PostPosted: Fri Dec 30, 2005 4:40 am
by rustyslide
>>I think I'm going to look into recreating a simple
>>12AX7/EL84 or 12AX7/6L6 (ie, one of each) amp circuit with
>>FETs and sticking it in a very portable but nothing-special
>>practice amp (that's also way too loud) I bought in the
>>fall.
>
>That's certainly doable, but you'll have to have the heavy
>and expensive parts of the tube amp (the transformers) if
>you're going to recreate such a tube amp circuit. Might as
>well do it with tubes and be done. Or buy a Valve Junior,
>which has got the single 12AX7 and EL84, and costs around
>$120 US.

The Valve Junior is great (convinced a friend to buy one), but it's too big and heavy for what I want - I want something that will fit in my nice load-distributing backpack for when I hit the road (via transit bus, Greyhound or just walking), that doesn't sound like crap. If my lovely Tech 21 amp was smaller everything would be fine - it's 4" wider, 2" taller and a bit deeper than it needs to be. I suppose I could create a new cabinet for it.

I found [link:www.runoffgroove.com/ruby.html|this schematic] in my travels, but it looks a little simple compared to the schematic I've seen for the Epi.

RE: Amp Knockdown Switches

PostPosted: Fri Dec 30, 2005 4:48 am
by bighollowtwang
[updated:LAST EDITED ON Dec-29-05 AT 11:50 PM (EST)]>I want
>something that will fit in my nice load-distributing
>backpack for when I hit the road (via transit bus, Greyhound
>or just walking), that doesn't sound like crap.

I can whole-heartedly recommend the Silvertone 1340:

http://www.harpamps.com/silvertone/1340.jpg

Same thing as the Harmony H-191:

http://www.harpamps.com/silvertone/h-191.jpg

Essentially the same circuit and the same tube layout as the 1949-1953 Fender Champion 600 (pre-"Champ"). I have the Silvertone version and I can assure you it doesn't sound remotely like crap. 14" tall, 12" wide, 8" deep, weighs about as much as a steel bodied resonator guitar. You can find 'em on ebay for about $200.

RE: Amp Knockdown Switches

PostPosted: Fri Dec 30, 2005 5:05 am
by rustyslide
>Essentially the same circuit and the same tube layout as the
>1949-1953 Fender Champion 600 (pre-"Champ"). I have the
>Silvertone version and I can assure you it doesn't sound
>remotely like crap. 14" tall, 12" wide, 8" deep, weighs
>about as much as a steel bodied resonator guitar. You can
>find 'em on ebay for about $200.

Unfortunately that's pretty much the size of my Tech 21 amp. My backpack can probably fit a 7" x 9" footprint. Looks cool though. Damnit.

Just found a Sansamp GT2 pedal (the guts of the Tech 21 amp) going for a low price on eBay, probably worth a shot.